You may be getting less power throughout your facility than you think, thanks to something known as power factor. Power factor affects power management, especially if you’re running very high power equipment such as an uninterruptible power supply (UPS). Power factor also affects accurate cost measurement since you are effectively getting less usable power than you’re paying for. So it pays to understand power factor and how it affects your operations.
Power factor definition
Power factor is the ratio of true power (actual power consumed) to apparent power (total power delivered). In a Direct Current (DC) power system, the voltage is constant and the power factor is generally a non-issue. With Alternating Current (AC), there is an inherent drop in usable power that occurs due to the way AC works.
Power factor explained
Most data centers operate on Alternating Current (AC) power, which flows in two directions. The current within an AC system flows in one direction, reverses, flows backward and then forward again 50 or 60 times a second, depending on what country you’re in. That means that voltage and current in an AC system fluctuate back and forth between positive and negative. Since the power being delivered is the product of voltage and current, it will fluctuate as well.
If voltage and current were perfectly synchronized you could simply multiply the average voltage by the average current to calculate the correct average power being delivered. Unfortunately, there are situations where voltage and current get out of sync -- voltage is high but current is low, or vice versa. This is why you may be getting less power than you think.
Picture a freshly-poured draft beer. You ordered a pint and paid for a pint, yet you won’t get a full pint to drink because there will be some frothy foam at the top. The more foam, the less beer, which means less value for your money.
The difference between apparent and true (or real) power
Here's how it works in your data center
- Apparent power: The consumption you’re paying for -- equivalent to our entire glass of beer with foam. Apparent power is expressed in VA (volt-amperes) or kVA. The apparent power rating (VA rating) represents the highest theoretical power that a piece of equipment can deliver or consume without exceeding either voltage or current ratings -- but only under perfect conditions.
- True power: The amount of power actually used by equipment to function -- equivalent to the beer. A unit of power is called a watt; true power is expressed in W (watts) or kW (kilowatts). In reality the true power is usually going to be less than the apparent power.
- Reactive power: The amount of power delivered but not consumed in a useful way -- equivalent to the foam.
Power factor identifies value received
A power factor of 1.0 is considered ideal because it occurs when voltage and current are completely in sync. This is unlikely in an AC power system, so your power factor will be lower than 1.0. The lower it is, the more inefficient (wasteful) your power consumption.
In order to precisely measure power, voltage and current must be measured and multiplied at exactly the same instance, many thousands of times a second. That’s how power monitoring equipment measures “true power.”
In a data center with modern power conditioning equipment and power supplies, you will rarely see circuits to IT devices with a power factor lower than 0.9. However, this is significant enough to make true power measurement essential when it comes to cost measurement and for managing very high power devices.
Reference our Volts, Amps, Watts, Watt-hours and Cost post for a refresher on electricity.
Email firstname.lastname@example.org if you need help measuring your true power. Packet Power wireless monitoring solutions are easy to install, affordable and accurate.